Enhanced brightness light emitting device spot emitter

ABSTRACT

The amount of usefully captured light in an optical system may be increased by concentrating light in a region where it can be collected by the optical system. A light emitting device may include a substrate and a plurality of semiconductor layers. In some embodiments, a reflective material overlies a portion of the substrate and has an opening through which light exits the device. In some embodiments, reflective material overlies a portion of a surface of the semiconductor layers and has an opening through which light exits the device. In some embodiments, a light emitting device includes a transparent member with a first surface and an exit surface. At least one light emitting diode is disposed on the first surface. The transparent member is shaped such that light emitted from the light emitting diode is directed toward the exit surface.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Division of U.S. patent application Ser.No. 10/283,737, filed on Oct. 29, 2002, and incorporated herein byreference.

BACKGROUND

[0002] 1. Field of Invention

[0003] The present invention relates generally to increasing thebrightness of a light emitting diode light source.

[0004] 2. Description of Related Art

[0005]FIG. 1 illustrates a lens 12 transmitting light generated by alight source 10 such as a light emitting diode. A key issue in designinglight sources to be used with optical systems comprised of passiveoptical imaging elements, such as lens 12, is illustrated in FIG. 1.Only light emitted from the source area that is consistent with theoptical invariant or etendue of lens 12 can be usefully focused onto thetarget area 20 (for example, a transparent microdisplay). The etendue ofa given optical system is defined as:

E=∫∫(cos θ)dAdΩ  (1)

[0006] where θ is the angle between the normal to the surface element dAand the centroid of the solid angle element dΩ. Etendue is a geometricproperty of the optics related to the divergence and cross-sectionalarea of the beam. The etendue cannot be decreased for if it were, theenergy density at the image could exceed that of the source, violatingthe second law of thermodynamics.

[0007] Source 10 may be, for example, a light emitting diode (LED),which emits light in all directions from both the top and side surfaces.As illustrated in FIG. 1, only light 16 emitted from the center of thetop surface of source 10 and within the cone accepted by the lens can befocused on the target 20. Light 14 emitted from the sides of lightsource 10, emitted from the top of source 10 far from lens 12, andemitted near lens 12 but at an angle outside the etendue-limit, is notutilized by lens 12, and is lost. In the case of a light emitting diodelight source 10, as the area of source 10 increases, in general thetotal light emitted from source 10 may also increase. However, theetendue of lens 12 imposes a maximum value on the amount of light fluxthat an optical system using lens 12 can utilize, regardless of howlarge light source 10 is made.

[0008] There are several ways to increase the amount of usefullycaptured light in an optical system. First, a lens with a largerdiameter 20 may be used. However, as the diameter of a lens increases,the cost of the lens increases. Thus, it is desirable to limit the sizeof the lenses in an optical system, in order to control the cost.

[0009] Second, the light flux per unit area of the light source may beincreased. In the case of a light emitting diode light source, theamount of light generated per unit area is generally proportional to theelectrical current density in the light generating layers of the device.Thus, the light per unit area may be increased by increasing the currentdensity. However, the efficiency of light emitting diodes usually fallsat high current densities due to, for example, heating effects,saturation in the light emitting layers of the charge carriers thatrecombine to produce light, or the loss of confinement of the chargecarriers that recombine to produce light. The loss of light generatingefficiency at high current density limits the amount of light generatedper unit area that can be created in a light emitting diode.

SUMMARY

[0010] In accordance with embodiments of the invention, the amount ofusefully captured light in an optical system may be increased byconcentrating light in a region where it can be collected by the opticalsystem.

[0011] In some embodiments, a light emitting device includes asubstrate, a plurality of semiconductor layers overlying the substrate,and a contact disposed on a first surface of the plurality ofsemiconductor layers. Light is extracted from the device through thefirst surface. A reflective material overlies a portion of the firstsurface and has an opening through which light exits the device.

[0012] In some embodiments, a light emitting device includes atransparent member with a first surface and an exit surface. At leastone light emitting diode is disposed on the first surface. Thetransparent member is shaped such that light emitted from the lightemitting diode is directed toward the exit surface. In some embodiments,the transparent member has two surfaces that form a wedge, with the apexof the wedge opposite the exit surface, and two parallel surfaces. LEDsare disposed on the two surfaces that form a wedge, and the two parallelsurfaces are coated with reflective material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 illustrates an optical system including a light emittingdiode, a lens, and a target image area.

[0014]FIG. 2 illustrates a top view of a light emitting device accordingto embodiments of the present invention.

[0015]FIG. 3 illustrates a cross sectional view of a light emittingdevice including a reflective layer, according to an embodiment of thepresent invention.

[0016]FIG. 4 illustrates a cross sectional view of an alternateembodiment of a light emitting device including a reflective layer thatcovers both the sides as well as a portion of the top of the chip.

[0017]FIG. 5 illustrates a cross sectional view of a light emittingdevice including an optical element, according to an embodiment of thepresent invention.

[0018]FIG. 6 illustrates a top view of the device shown in FIG. 5.

[0019]FIG. 7 illustrates a cross sectional view of a light emittingdevice including a fluorescent material, according to an embodiment ofthe present invention.

[0020]FIG. 8 illustrates a cross sectional view of a light emittingdevice including a dome, according to an embodiment of the presentinvention.

[0021]FIG. 9 illustrates an exploded view of a packaged light emittingdevice.

[0022]FIG. 10 illustrates a cross sectional view of an alternateembodiment of a light emitting device.

[0023]FIGS. 11A and 11B illustrate a device with LEDs disposed on thesides of a transparent wedge.

[0024]FIG. 12 illustrates a red LED disposed on a side of a transparentwedge with a dichroic filter.

DETAILED DESCRIPTION

[0025] In accordance with embodiments of the invention, the amount oflight captured in an optical system may be increased by directing lightfrom the source into the etendue-limit of the optical system so that itcan be captured by the optical system. The light source may be asemiconductor light emitting device such as a light emitting diode.Embodiments of the invention are applicable to semiconductor lightemitting devices of various materials systems, including, for example,III-V systems such as III-nitride, III-phosphide, and III-arsenide, andII-VI systems. Further, embodiments of the invention are applicable toany semiconductor light emitting devices where the device layers andsubstrate are reasonably transparent to light, including devices havingboth contacts formed on the same side of the device, such as flip-chipand epitaxy-up devices, as well as devices having contacts formed onopposite sides of the device.

[0026]FIG. 2 illustrates a top view of a light emitting device, the sideof the device through which light is extracted and which is oftenadjacent to optics such as lenses. A portion of the device faceillustrated in FIG. 2 is covered by a reflective layer 22. Light onlyescapes from region 24, which may correspond to the maximum source areaconsistent with the etendue-limit of a lens in the optical system.

[0027]FIG. 3 illustrates a cross sectional view of a light emittingdevice. A layer of first conductivity type 26 is formed on a substrate25. If the device shown in FIG. 3 is a III-nitride light emitting diode,first conductivity type layer 26 may be an n-type III-nitride layer andsubstrate 25 may be sapphire, SiC, GaN, or any other suitable substrate.A light emitting region 28, also referred to as the active region, isformed on first conductivity type layer 26, then a layer of secondconductivity type 32 is formed on active region 28. A first contact 35is connected to the layer of first conductivity type and a secondcontact 34 is connected to the layer of second conductivity type. Atleast one of contacts 34 and 35 may be reflective. Interconnects 36connect the light emitting diode to a submount. Interconnects 36 may be,for example, solder bumps or gold bumps.

[0028] A reflective layer 22 prevents light from escaping the deviceoutside an area that matches the etendue-limit of a lens in the opticalsystem. The semiconductor layers in an LED are typically quitetransparent to light at the emission wavelength. Thus, a light ray 30which would normally escape substrate 25 outside the etendue-limit oflens 12 is reflected off reflective layer 22, transmitted through layers26, 28, and 32 without absorption, then reflected off reflective contact34 until ray 30 escapes substrate 25 in region 24, the region of thesurface of the light emitting diode that is not covered by reflectivelayer 22. Reflective layer 22 and reflective contacts 34 and 35 createan optical cavity where light generated from active region 28 outsidethe etendue-limit of lens 12 is reflected back and forth until the lightreaches region 24, where it can be utilized by lens 12.

[0029] In one embodiment, reflective layer 22 may be, for example, ametal having a reflectivity greater than 90%. Optical modeling hasdemonstrated that using a metal having a reflectivity greater than 98%yields up to a 50% gain in light collected by the optical system.Examples of suitable metals are as silver, aluminum, rhodium, and gold.The reflective metal may be selected based on the material on which itis to be deposited, or the wavelength of the light it is to reflect. Forexample, gold is highly reflective of light in the red or infra-redwavelength ranges.

[0030] In another embodiment, reflective layer 22 may be, for example, anon-specular (white) highly-reflective layer. One example of a suitablematerial is a white powder or paint containing barium sulfate, such asWhite Reflectance Coatings available from Munsell Color Services of NewWindsor, N.Y. The non-specular layer may be applied by, for example,painting or electrophoretic deposition.

[0031]FIG. 4 illustrates an alternative embodiment of the invention,where reflective layer 22 extends down over the sides of the lightemitting diode. A ray of light 38 which would normally be emitted outthe side of the light emitting diode is reflected off reflective layer22 on the side of the device, then reflected off reflective layer 22 onthe top of the light emitting diode, then reflected off reflectivecontact 34 until it escapes through substrate 25 in region 24. Inembodiments where reflective layer 22 is insulating, such as whenreflective layer 22 is a non-specular paint layer, reflective layer 22may be deposited directly on the sides of the light emitting diode. Inembodiments where reflective layer 22 is conducting, such as whenreflective layer 22 is a reflective metal, a dielectric layer must firstbe deposited over the sides of the light emitting diode, to preventreflective layer 22 from creating a short between the layer of firstconductivity type and the layer of second conductivity type.Alternatively, if reflective layer 22 is conducting, it may cover onlypart of the sides of the light emitting diode such as the sides ofsubstrate 25, so as not to create a short.

[0032] Different materials may be used to create reflective layer 22 indifferent areas on the light emitting diode. For example, a reflectivemetal may be used on the top of the diode, while an insulatingnon-specular material may be used on the sides of the diode.

[0033]FIG. 5 illustrates a device with an optical structure 41 bonded tothe light emitting diode. The top of optical structure 41 is shaped toreflect light emitted outside the etendue-limit of an optical systemback into the device, such that it can be extracted from region 24.Optical structure 41 includes a transparent material 44, and areflective layer 42 formed over some edges of transparent material 44.Reflective layer 42 may be, for example, a reflective metal such asthose described above in the text accompanying FIG. 3. Transparentmaterial 44 may be index-matched to the adjacent material, substrate 25in FIG. 5. A light ray 46 outside the etendue-limited area (region 24)is transmitted through transparent material 44, then reflected offreflective layer 42. Light ray 46 reenters the device, where it isreflected off contact 34, then escapes substrate 25 in region 24.Optical structure 41 may be used in conjunction with reflective layer22, shown in FIGS. 2-4. For example, optical structure 41 may be used onthe top surface of the device, while a reflective layer 22 is used onthe sides of the device.

[0034]FIG. 6 illustrates a top view of the device shown in FIG. 5. Threeconcentric circles 49 correspond to the ends of portions of reflectivelayer 42 of optical structure 41, shown in FIG. 5. Note the light-escapearea 24 need not be circular. Light-escape area 24 may be square,rectangular, oval, or any other shape. For example, if the light fromthe device is to be coupled into a long, thin lightguide, light-escapearea 24 may be long and thin. The boundary of the light emitting diodeis shown by reference 45.

[0035]FIG. 8 illustrates a cross sectional view of a device with analternative optical structure, including a transparent dome 60. Portionsof dome 60 are covered by a reflective material 61 such that light isonly emitted from dome 60 in the light-escape area 24. The device shownin FIG. 8 differs from the device shown in FIG. 5 in that dome 60 coverslight-escape area 24, while optical structure 41 of FIG. 5 has a holecorresponding to light-escape area 24. As is clear to one of skill inthe art, any suitable optical structure may be used according toembodiments of the invention, not just the dome structure shown in FIG.8 or the fresnel-like structure shown in FIG. 5.

[0036]FIG. 7 illustrates an embodiment of the invention where awavelength-converting material is deposited over light-escape area 24.Material 50 may be, for example, a fluorescent material such as phosphordeposited over the region of substrate 25 that is left exposed byreflective layer 22. Though FIG. 7 shows material 50 deposited over thedevice illustrated in FIG. 3, any of the other embodiments describedabove may be combined with wavelength-converting material 50. If activeregion 28 is III-nitride such that the emission from active region 28 isblue, material 50 may be a Ce-doped Yttrium Aluminum Garnet (YAG)phosphor, which absorbs blue emission and emits yellow light. Yellowlight from material 50 may mix with blue light from active region 28such that the light from region 24 appears as an intense white lightsource.

[0037]FIG. 10 illustrates an embodiment of the invention where the lightemitting diode is an epitaxy-up device instead of a flip chip device.The device shown in FIG. 10 has transparent contacts 82 instead ofreflective contacts. Light is extracted through the contacts. In theembodiment illustrated in FIG. 10, reflective material 22 is a metal, towhich wire bonds 80 are connected for making electrical contact totransparent contacts 82. The bottom of substrate 25 may be coated with areflective material (not shown).

[0038]FIG. 9 is an exploded view of a packaged light emitting device. Aheat-sinking slug 100 is placed into an insert-molded leadframe 106. Theinsert-molded leadframe 106 is, for example, a filled plastic materialmolded around a metal frame that provides an electrical path. Slug 100may include an optional reflector cup 102. The light emitting device die104, which may be any of the devices described above, is mounteddirectly or indirectly via a thermally conducting submount 103 to slug100. An optical lens 108 may be added.

[0039]FIGS. 11A and 11B illustrate another device for directing lightemitted by a source into the etendue-limit of an optical system. Thedevice of FIG. 1 IA has multiple light sources 91 and 92 disposed alongthe edges of a transparent wedge 96. Wedge 96 may be, for example,sapphire, glass, acrylic, silicone, or any other suitable materialcapable of maintaining transparency when exposed to the light and heatemitted by light sources 91 and 92. The light sources may be LEDsmounted on submounts 94 and attached to the wedge by, for example,gluing, pressing, or bonding. The size and shape of an exit surface 95of transparent wedge 96 is selected to correspond to the etendue-limitof an optical system (not shown). The sides of the wedge with LEDs neednot be completely covered with LEDs. Portions of the sides not coveredwith LEDs may be coated with a reflective coating. The sides of thewedge without LEDs may also be coated with a reflective coating 93. Thecoated sides 93 and LEDs 91 and 92 create a tapered cavity with only oneopening, the exit surface. Since the semiconductor layers and substratein LEDs 91 and 92 are transparent to the emitted light, the light isreflected off sides 93 and the reflective contacts of LEDs 91 and 92until the light exits the exit surface, as illustrated in FIG. 11B. Theshape of the wedge directs all light to the exit surface. In someembodiments, exit surface 95 has the same dimensions as a single LED,though it may be larger or smaller.

[0040] A wedge with eight perfectly reflective LEDs and an exit surfacethe same size as a single LED produces eight times more light in thesame area as a single LED. Real devices are generally not perfectlyreflective. A wedge with eight LEDs that are 85% reflective will producefour to five times more light in the same area as a single LED.

[0041] The wedge device illustrated in FIGS. 11A and 11B may be suitableas a source in many applications requiring high brightness, including,for example, projectors, car headlights, fiber optics, and theaterlights. Homogeneous illumination of the exit surface makes the wedgeparticularly suitable to projection applications. The size and shape ofthe exit surface of the wedge may be tailored to individualapplications.

[0042] In some embodiments, LEDs of different colors are mounted on theedges of wedge 96, such that the light emitted from the exit surface isa mixture of the different colors. For example, red, blue, and greenLEDs may be used such that the mixed light exiting the exit surfaceappears white. Red LEDs are generally not very reflective of blue andgreen light. Thus, a dichroic materal may be used in embodimentsincluding red LEDs, as illustrated in FIG. 12. A red flip chip LED 102with a reflective contact 101 is mounted on wedge 96 with a dichroicfilter 103 between the LED and the wedge. Red light 104 emitted from theactive region of LED 102 passes through dichroic filter 103. Red light105 inside the wedge passes through dichroic filter 103, is reflectedoff contact 101, then passes through dichroic filter 103 again toreenter the wedge. Blue or green light 106 is reflected by dichroicfilter 103. The dichroic material is selected to reflect blue and greenlight and to transmit red light.

[0043] Having described the invention in detail, those skilled in theart will appreciate that, given the present disclosure, modificationsmay be made to the invention without departing from the spirit of theinventive concept described herein. Therefore, it is not intended thatthe scope of the invention be limited to the specific embodimentsillustrated and described.

What is being claimed is:
 1. A light emitting device comprising: a lightemitting diode comprising a substrate, a plurality of semiconductorlayers overlying the substrate, and a contact disposed on a firstsurface of the plurality of semiconductor layers, wherein light isextracted through the first surface; and a reflective material overlyinga portion of the first surface, the reflective material having anaperture through which light exits the device.
 2. The light emittingdevice of claim 1 wherein the contact is at least partially transparent.3. A light emitting device comprising: a transparent member having afirst surface, a second surface, and an exit surface; at least a firstlight emitting diode disposed on the first surface; and one of a secondlight emitting diode and a reflective coating disposed on the secondsurface; wherein the transparent member is shaped such that lightemitted from the at least one light emitting diode is directed towardthe exit surface.
 4. The light emitting device of claim 3 furthercomprising a reflective coating disposed on a portion of the firstsurface not covered by the first light emitting diode.
 5. The lightemitting device of claim 3 wherein the second surface is coated with areflective coating, the device further comprising: a third surface; afourth surface coated with a material reflective of light emitted fromthe first light emitting diode; and a second light emitting diodedisposed on the third surface; wherein the first and third surfaces forma wedge with an apex opposite the exit surface, and the second andfourth surfaces are substantially parallel.
 6. The light emitting deviceof claim 5 wherein the first light emitting diode and the second lightemitting diode each emit light at a different wavelength.
 7. The lightemitting device of claim 5 wherein the second light emitting diode emitsred light, the device further comprising a dichroic material disposedbetween the second light emitting diode and the third surface.
 8. Thelight emitting device of claim 7 further comprising a third lightemitting diode disposed on one of the first surface and the thirdsurface, wherein one of the first and the third light emitting diodesemits green light and the other of the first and the third lightemitting diodes emits blue light.
 9. The light emitting device of claim5 wherein the first light emitting diode and the second light emittingdiode each emit light of substantially the same color.
 10. The lightemitting device of claim 3 wherein the first light emitting diodecomprises: a transparent substrate; a plurality of semiconductor layersthat are transparent to light emitted by the first light emitting diode;and a contact reflective of light emitted by the light emitting diode,wherein the plurality of semiconductor layers are disposed between thetransparent substrate and the contact.
 11. The light emitting device ofclaim 10 further comprising a submount attached to the contact, whereinthe first light emitting diode is mounted on the first surface such thatthe transparent substrate is closest to the first surface.
 12. The lightemitting device of claim 3 wherein the first surface and the exitsurface are not parallel.
 13. The light emitting device of claim 3wherein the transparent member is selected from the group consisting ofsapphire, glass, acrylic, and silicone.